In a mobile cellular network, geological communicable areas are divided into many cells, and each of the cells corresponds to a single base station. When power is supplied to a mobile station, the mobile station searches for a base station as a communication destination. In most cases, a cell search function is performed based on a cell-specific reference signal, which is periodically transmitted, or a preamble signal. In a basic method, the search is comprehensively performed by detecting all reference signals and then acquiring the best base station, based on some given criteria or factors, to serve as a communication destination.
There is a proposed method used in a mobile communication system in which a macrocell comprises a smaller microcell (see, for example, Japanese Laid-open Patent Publication No. 2007-514367). The above-described method includes a process for establishing an uplink communication cell boundary between a macrocell and a microcell, and a process for establishing a downlink communication cell boundary differently from the uplink communication cell boundary between the macrocell and the microcell. The uplink communication cell boundary is established to be wider than the downlink communication cell boundary. The downlink communication cell boundary is established by inclining a downlink antenna beam of the base station in association with the microcell transmitting a broadcast signal and by reducing a coverage area of the broadcast signal.
There is a proposed base station that includes a plurality of tilt beam antennas to cover substantially the same communication area and a beam tilt angle control unit that changes a tilt angle of at least one of the tilt beam antennas to make the tilt angle different from the tilt angles of the other beam antennas. The above-described base station performs rake combining on a level received by using at least two tilt beam antennas on an uplink line and transmits signals by using at least one tilt beam antenna with a higher reception level on a downlink line (see, for example, Japanese Laid-open Patent Publication No. 2007-28091).
With which cell the mobile station prefers to communicate differs according to whether the signal is the uplink or the downlink. The above-described state may be called “uplink-and-downlink asymmetric state.”
The uplink-and-downlink asymmetric state may occur if, for example, the base station devices, which are arranged respectively in the adjacent cells, have different transmission power. Here, it is assumed that the base station devices having different transmission power are arranged in the adjacent two cells and that the mobile station is positioned in a position that is slightly closer to the base station device whose transmission power is relatively small than the base station device whose transmission power is relatively large. In this case, as for the downlink, reception power received from the base station device whose transmission power is relatively large is larger than the reception power of the base station device whose transmission power is relatively small. Therefore, it is preferable that the mobile station communicates with the base station device whose transmission power is relatively large.
On the other hand, as for the uplink, the base station device, which is closer to the mobile station and has the relatively small transmission power, has the reception power that is larger than the reception power of the base station device whose transmission power is relatively large. Accordingly, it is preferable that the mobile station communicates with the base station device whose transmission power is relatively small.
If the uplink-and-downlink asymmetric state occurs, interference power received by the other cell is larger than the reception power of the uplink received by the communicating cell. This may cause deterioration of the communication quality in the uplink.